Let's start with what has gone wrong. The mower has been run with the front chassis rail broken in two across the front right engine mount, resulting in the engine thrashing about, which would maltreat the crankshaft and clutch in particular, but also the self-aligning bearing in the side plate behind the drive sprocket. If the engine side of the clutch can rotate without the crankshaft rotating, either the key is sheared or missing, or the keyway has torn out of the crankshaft or clutch. (It's also possible the crankshaft has snapped off inside the clutch.) Let's hope it's either a sheared key or the engine-side clutch-half has a torn keyway, because then there might be no damage to the engine, and the clutch half would probably be trashed anyway from the flogging it has suffered. If it is the crankshaft that has the keyway damage or is broken off, you need to replace the crankshaft with a second-hand one, or replace the engine. It is also possible the crankshaft has suffered the beginning of a fatigue failure at the step that coincides with the crankcase cover oil seal: it was being treated abominably when it was being run by the previous owner continuously with that broken rail.
The engine identification numbers are stamped on the engine's cooling air cowl. If someone has fitted a cowl from some other Briggs and Stratton engine, then the information it provides will not apply to the engine it is now on. One way to tell would be to scrape some paint off in an inconspicuous location on the crankcase. If it has orange underneath, the machine has been falsified, probably deliberately, by a previous owner, and the engine is older than the cowl. If that is the case its power output is unclear, but if it is an 8 cubic inch orange engine it would be 2.5 HP. If it is a 60102 rather than a 80202 it would be 2.0 HP. (I think that cowl fits both 60102 and 80202 engines, though I'm not completely sure it fits a 60102). Nobody has posted whether that is a 20" mower or a 17". If it is a 20", it needs 3 HP. As I recall the 17" was in production for quite a while with 2.5 HP, and the 14" originally had 2 HP, but Deejay can put us right on that.
I suggest that the first thing you need to do is the paint-scraping test to find whether the engine has been falsified with the wrong cowl and paint colour, and the second thing is to unbolt the engine, split the clutch, then remove the engine side of the clutch from the crankshaft, so you can inspect the keyway in both the crankshaft and the clutch.
